1. Field of the Invention
The field of the present invention relates to a hydroplaning detecting system for detecting a hydroplaning phenomenon generated when a vehicle travels over a sheet of water on a road surface at a high speed.
2. Description of the Related Art
The term "hydroplaning phenomenon" refers to a phenomenon that occurs when the vehicle travels over a water screen such as a puddle at a high speed (in general, at least 80 km/hr), in which a wheel is temporarily lifted up from the road surface, and the friction coefficient between the wheel and the road surface is rapidly reduced. The hydroplaning results when the water screen enters into a wedge-shaped space between the road surface and the wheel at high speed, thereby permitting the wheel to be urged away from the road surface by the dynamic pressure of the water screen.
As shown in FIG. 5, hydroplaning is generated within an extremely short time, much less than 1 second and, during this time, the rotational speed of the wheel is reduced rapidly and then restored instantaneously. Specifically, when the wheel is lifted up from the road surface, the wheel is rapidly reduced in rotational speed (in general, by at least several km/hr) by the resistance of the water screen entering between the wheel and the road surface. The wheel then contacts with the road surface again, and as a result, the original speed is restored. Thus, the pattern of speed in this case has a shape of a downwardly indented wave. When the water screen extends along the road surface through a long distance, such hydroplaning may be generated in a repeated manner at short time intervals, and during this time, braking and steering performances may be affected.
A conventionally known hydroplaning detecting technique is based on rapid indented wave-like reduction in rotational speed of the wheel for detection of the generation of the hydroplaning, such as when the magnitude of a deceleration obtained by differentiation of the rotational speed of the wheel exceeds a reference value, as described, for example, in Japanese Patent Application Laid-open No. 265172/88.
However, the rapid reduction in rotational speed of the wheel of the vehicle is generated not only when the hydroplaning phenomenon is produced, but also in other cases, for example, such as when the vehicle travels on a bad road, or when a wheel rides over a protrusion or a step on a road surface (see FIGS. 6A to 6D). For this reason, with the above prior art technique using a differentiated value of the rotational speed of the wheel, it is difficult to reliably distinguish a deceleration of the wheel due to hydroplaning from a deceleration of the wheel due to other causes, and the accuracy of detection, therefore, has a limit.